Brayton Cycle

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The Closed Brayton Cycle

George Brayton (1830-1892), U.S. mechanical engineer and pioneer in the development of internal combustion engines, invented the continuous ignition combustion engine that later became the basis for the turbine engine. He began working on internal combustion engines in the 1870s. The Patent Office identifies George Brayton's 1872,.2-cycle engine as a hot-air engine that ran quietly with petroleum fuel. The Brayton Cycle became the basis for all gas turbine engines and he is believed to have manufactured the first gas turbines commercially in Providence, Rhode Island. For a while his hot air engine became the preferred engine of the American auto industry.

There are many forms of Brayton Cycle, ranging from the simple open cycle used in gas turbine and jet engines, to reverse Brayton cycles used in cooling systems and to the closed cycle with external combustion, similar in concept to the Stirling cycle.

During the Cold War it became necessary to orbit satellites with long-duration missions that required more power than could be obtained from radiated sun energy. The solution adopted by the US and the Soviets was to use nuclear heat sources for power generation. An intensive research program followed to find and develop the best methods to convert the heat energy into electric power. One such method was the Radioisotopic Thermoelectric Generator (RTG) the other for higher levels of power was the closed Brayton cycle. Thus it happened that on April 3^rd, 1965 SNAP 10-A was launched. The satellite shut down after 43 days, because of a spacecraft malfunction, not related to the reactor or power system. This reactor is currently in a nuclear-safe storage orbit with an estimated life of three-thousand years.

As one might imagine, for long-term missions, a great deal of development went into ensuring the reliability of the closed-Brayton cycle power generator. However, subsequently that technology was not widely exploited. Now, with the development of mass produced commercial turbochargers and advances in heat exchanger design and manufacturing, it is has become possible to build closed-Brayton cycle generators with extreme reliability, long endurance, and economic characteristics most favorable for use in CHP systems.

Creative Energy Concepts has proceeded with the design, development and demonstration of such generators and find them to be economically and physically competitive with existing high efficiency Diesel generators at industrial rates of production and to pose very low technical risks. Furthermore, unlike a Diesel, since a closed Brayton cycle employs external combustion, it can be designed to produce ultra low emissions.

Its greatest advantage though lies in the fact that it is a more efficient thermal cycle with the unusual ability to produce very good part load thermal efficiencies – which is its most valuable attribute from the standpoint of its use in CHP systems.

Air at its coolest enters the compressor. Compressed air is passed through the recuperator to be heated with exhaust air from the turbine and heated further to 1,800ºF by radiation and convection from the external combustor. The hot air expands through the turbine which drives the compressor and generator. The turbine exhaust is partially cooled in the recuperator and further cooled by a heat sink heat exchanger before entering the generator, to repeat the cycle.

The closed loop Brayton cycle is the best solution to the energy system needs of the target markets which CEC is focusing on. These target applications have thermal and electrical loads that are subject to large swings. Hence, the ideal energy system has to operate efficiently under fractional load conditions and must be able to accommodate thermal load swings which do not coincide with electrical load changes.

The Open Cycle

The open Brayton cycle as used in conventional gas turbines, is open to the atmosphere at the inlet to the compressor (after cooling the generator in this case), and uses an in-line combustor to heat the gas entering the turbine. The key differences are that:

the simple open cycle does not offer the opportunity of controlling the density of the air passing through it, and
it uses internal combustion, which means that the fuel has to be compressed to be injected into the compressed air stream.

Part Load Efficiency

In the closed cycle, the air pressure entering the loop can be increased or decreased by adding or bleeding off air in the loop, which effectively means controlling the density of the working fluid in the cycle. This permits the cycle to operate at design efficiency even as the load is diminished, by reducing the pressure (density) in the loop. The two stage CEC turbo generator, shown in red, maintains close to 40% peak efficiency all the way from 8% to 100% output.

Because the air pressure entering an open cycle gas turbine, as shown in brown, cannot be controlled, its thermal efficiency drops off as the load is reduced below the design point. For this reason, conventional gas turbines are fundamentally base load machines and are best operated continuously at full load.

In CHP situations without grid connection, it depends on the available heat load, whether the open cycle part load losses can be put to good use or not. In the majority of real world conditions, the generator is forced to operate at part load fir extended periods each day, which gives the closed loop cycle a profound advantage.

Non-coincident Load Capability

In CHP use, most on-site generators have the opposite problem of having insufficient heat to meet heating loads in the presence of low electricity demand. This may require a separate furnace to augment the heat from the generator exhaust as when Diesel generators are used.

The closed cycle has the advantage that it uses an external combustor and during non-coincidental load conditions, it bypasses the the recuperator to the extent necessary to provide the extra heat.